Patent Document 1, for example, discloses a gas sensor element having a composite layer in which a solid electrolyte body (electrolyte portion to be described herein later) and an insulation member (insulation portion to be described herein later) exist. In Patent Document 1, the dimensional difference between the thickness of the solid electrolyte body and the thickness of the insulation member is rendered equal to or less than the thickness of an electrode (conductor layer). Thus, a portion of the electrode laminated on the solid electrolyte body and a portion laminated on the insulation member are at least partially in contact with each other, whereby breaking of the electrode at a portion on the boundary between the solid electrolyte body and the insulation member can be prevented.